The related application identified above discloses a frequency domain equalizer for vestigial sideband (VSB) decoders implemented within a single integrated circuit multi-standard demodulator also including an orthogonal frequency division multiplexing (OFDM) decoder. Rather than a frequency domain implementation of a time domain finite impulse response (FIR) filter, which is computationally intensive, the frequency domain equalizer disclosed employs a least square cost function which is memory intensive in adaptive inverse channel estimation. As a result, the hardware required for adaptive inverse channel estimation may be mapped to the hardware employed for (memory intensive) OFDM decoding.
From simulation results, the performance of the frequency domain equalizer disclosed in the related application is expected to be at least as good as currently implemented VSB equalizers, and in some cases even better, particularly for noisy non-minimum phase channels (where decision feedback equalizers suffer from convergence to a local minims if the length of the forward taps is short), long delay dispersions and co-channel interference. In addition, performance of the frequency domain equalizer disclosed is hardly affected by the location of a spike initialization (the center tap).
Even though performance of the frequency domain equalizer is at least as good as if not better than the performance of a typical standard time domain decision feedback equalizer (DFE) under the circumstances described, use of the frequency domain equalizer alone may not be adequate to solve the equalization problem of vestigial sideband (VSB) modulation with eight discrete amplitude levels (8-VSB). The frequency domain equalizer behavior approximates a finite impulse response (FIR) Wiener solution, which could be inadequate for noise channels that have deep notches.
On the other hand, a decision feedback equalizer (discussed generally in J. Proakis, Digital Communications, Third Edition) would theoretically exhibit better performance in such a situation if the decisions fed back to the equalizer are reliable and the length of the filters is sufficiently long. With reliable decisions, such as with a high signal-to-noise ratio (SNR) signal or using trellis decisions, the decision feedback equalizer would perform better than the frequency domain equalizer disclosed in the related application for some channels.
There is, therefore, a need in the art for a frequency domain equalizer which retains the advantages of adaptive inverse channel estimation while also obtaining the benefits of the feedback portion of a decision feedback equalizer with reliable decisions and a sufficiently long filter.